The present invention relates to touchpads, and in particular to methods for assembling lightweight, very thin, low cost touchpads.
A touchpad is one method for providing a pointing function to control a cursor or viewpoint on a computer. Alternately, the display itself can be a touchscreen, being activated when the user presses on the screen. Detection of the user finger or stylus has been done by a variety of different ways. Resistive tablets are one method that has been used, such as disclosed in U.S. Pat. No. 4,680,430. Another method uses surface acoustic waves. Yet another method uses strain gauges or pressure plates. Optical devices have also been used.
The most common type of commercially used touchpad device is a capacitive sensing device. One advantage of the capacitive-type device is the lower power consumption required and it doesn't require finger pressure. An example of such a device, which also discusses a number of other related technologies, is set forth in U.S. Pat. No. 5,495,077.
Touchpads are currently built into some laptop computers, typically being on a portion of a computer in front of the keyboard where the user would normally rest his or her hands. For a portable computer application, it is important that the touchpad unit consume little power, be compact, and be lightweight.
Typically, a touchpad requires a substrate with multiple layers. The layers include rows of conductive traces, followed by an insulating layer, followed by columns of conductive traces, followed by another insulating layer and a ground plane. By pressing on a particular portion of the array, the capacitance between the conductive lines and ground varies and can be detected. By sensing both rows and columns, the position of the changing capacitance can be pinpointed. A variety of electronic components are required in order to control and detect the signals from the sensor array.
One technology used in some electronics applications is a flexible, Mylar sheet upon which are deposited conductive ink. Such a sheet is thinner than a typical PC board, and thus provides size advantages. However, components cannot be soldered to a Mylar sheet without melting it, and thus conductive adhesives are required to attach components. Thus, electronic chips and discrete components can be attached to such a Mylar sheet.
One limitation in using a conductive glue or adhesive to attach components to the conductive ink traces on a Mylar sheet is that there must be sufficient spacing between the contacts or pads attaching to the sheet. Thus, greater spacing between leads of components, such as semiconductor chips, is required than would be required on a PC board. Thus, a tradeoff of using a Mylar sheet is that lower weight and a thinner profile can be obtained, but at the expense of requiring a greater area on which to place the components. One solution for semiconductor chips is to mount them in a larger package, with the package itself having a wider lead spacing which can then be attached to the Mylar sheet.
An approach used by Alps is to use a combination flexible sheet and circuit board at different layers for a touchpad device. Conventional touchpads use 4 layer PC boards. Instead of building all 4 layers of a touchpad from PC board material, a 2 layer PC board is used for the Alps touchpad. The flexible material is used for the upper 2 layers, and is glued to the PC board. This still gives a PC board which is the same size and is rigid, it does not provide a thin touchpad. Through holes are used to connect to the underlying PC layer, with conductive ink being placed on the flexible layer and over the through holes.